Mems device and a method of using the same

ABSTRACT

A method of using a MEMS gyroscope is disclosed herein, wherein the MEMS gyroscope comprised a magnetic sensing mechanism. A magnetic field is generated by a magnetic source, and is detected by a magnetic sensor. The magnetic field varies at the location of the magnetic sensor; and the variation of the magnetic field is associated with the movement of the proof-mass of the MEMS gyroscope. By detecting the variation of the magnetic field, the movement and thus the target angular velocity can be measured.

CROSS-REFERENCE

This U.S. utility patent application claims priority from co-pendingU.S. utility patent application “A HYBRID MEMS DEVICE,” Ser. No.13/559,625 filed Jul. 27, 2012, which claims priority from USprovisional patent application “A HYBRID MEMS DEVICE,” filed May 31,2012, Ser. No. 61/653,408 to Biao Zhang and Tao Ju. This U.S. utilitypatent application also claims priority from co-pending U.S. utilitypatent application “A MEMS DEVICE,” Ser. No. 13/854,972 filed Apr. 2,2013 to the same inventor of this U.S. utility patent application, thesubject matter of each of which is incorporated herein by reference inits entirety.

TECHNICAL FIELD OF THE DISCLOSURE

The technical field of the examples to be disclosed in the followingsections is related generally to the art of operation ofmicrostructures, and, more particularly, to operation of MEMS devicescomprising MEMS magnetic sensing structures.

BACKGROUND OF THE DISCLOSURE

Microstructures, such as microelectromechanical (hereafter MEMS) devices(e.g. accelerometers, DC relay and RF switches, optical cross connectsand optical switches, microlenses, reflectors and beam splitters,filters, oscillators and antenna system components, variable capacitorsand inductors, switched banks of filters, resonant comb-drives andresonant beams, and micromirror arrays for direct view and projectiondisplays) have many applications in basic signal transduction. Forexample, a MEMS gyroscope measures angular rate.

A gyroscope (hereafter “gyro” or “gyroscope”) is based on the Corioliseffect as diagrammatically illustrated in FIG. 1. Proof-mass 100 ismoving with velocity V_(d). Under external angular velocity Ω, theCoriolis effect causes movement of the poof-mass (100) with velocityV_(s). With fixed V_(d), the external angular velocity can be measuredfrom V_(d). A typical example based on the theory shown in FIG. 1 iscapacitive MEMS gyroscope, as diagrammatically illustrated in FIG. 2.

The MEMS gyro is a typical capacitive MEMS gyro, which has been widelystudied. Regardless of various structural variations, the capacitiveMEMS gyro in FIG. 2 includes the very basic theory based on which allother variations are built. In this typical structure, capacitive MEMSgyro 102 is comprised of proof-mass 100, driving mode 104, and sensingmode 102. The driving mode (104) causes the proof-mass (100) to move ina predefined direction, and such movement is often in a form ofresonance vibration. Under external angular rotation, the proof-mass(100) also moves along the V_(s) direction with velocity V_(s). Suchmovement of V_(s) is detected by the capacitor structure of the sensingmode (102). Both of the driving and sensing modes use capacitivestructures, whereas the capacitive structure of the driving mode changesthe overlaps of the capacitors, and the capacitive structure of thesensing mode changes the gaps of the capacitors.

Current capacitive MEMS gyros, however, are hard to achievesubmicro-g/rtHz because the capacitance between sensing electrodesdecreases with the miniaturization of the movable structure of thesensing element and the impact of the stray and parasitic capacitanceincrease at the same time, even with large and high aspect ratioproof-masses.

Therefore, what is desired is a MEMS device capable of sensing angularvelocities and methods of operating the same.

SUMMARY OF THE DISCLOSURE

In view of the foregoing, a method of measuring an angular velocity byusing a MEMS gyroscope is disclosed herein, the method comprising:generating a magnetic field that is associated with a movement of aproof-mass of the MEMS gyroscope, wherein the magnetic field varies at alocation of a magnetic sensor; detecting the variation of the magneticfield by the magnetic sensor; and extracting the angular velocity fromthe variation of the magnetic field.

In another example, a method of detecting a target angular velocity isdisclosed herein, the method comprising: providing a MEMS gyroscope thatcomprises a movable proof-mass, a magnetic source, and a magneticsensor, wherein the proof-mass is capable of moving in response to thetarget angular velocity under the Coriolis effect, and wherein themagnetic source is capable of generating a magnetic field that varieswith the movement of the proof-mass, and wherein the magnetic sensor iscapable of detecting the magnetic field from the magnetic source;generating the magnetic field by the magnetic source; detecting avariation of the magnetic field by the magnetic sensor; and extractingthe angular velocity from the variation of the magnetic field.

In yet another example, a method of measuring an angular velocity byusing a MEMS gyroscope is disclosed herein, the method comprising:detecting a background magnetic signal by a reference magnetic sensor ofthe MEMS gyroscope; locking the state of the reference sensor after thedetection of the background magnetic signal; generating a magnetic fieldby a magnetic source whose movement is associated with a proof-mass ofthe MEMS gyroscope; detecting the magnetic field from the magneticsource by a signal sensor; and calculating the angular velocity from thedetected magnetic field.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 diagrammatically illustrates the Coriolis effect in a MEMSstructure;

FIG. 2 is a top view of a typical existing capacitive MEMS gyroscopehaving a driving mode and a sensing mode, wherein both of the drivingand sensing mode utilize capacitance structures;

FIG. 3 illustrates an exemplary MEMS gyroscope having a magnetic sensingmechanism;

FIG. 4 illustrates a top view of a portion of an exemplaryimplementation of the MEMS gyroscope illustrated in FIG. 3, wherein theMEMS gyroscope illustrated in FIG. 4 having a capacitive driving modeand a magnetic sensing mechanism;

FIG. 5 illustrates a perspective view of a portion of another exemplaryimplementation of the MEMS gyroscope illustrated in FIG. 3, wherein theMEMS gyroscope illustrated in FIG. 5 having a magnetic driving mechanismfor the driving mode and a magnetic sensing mechanism for the sensingmode

FIG. 6 illustrates an exemplary magnetic driving mechanism of the MEMSgyroscope in FIG. 5;

FIG. 7 illustrates an exemplary magnetic source of the MEMS gyroscopeillustrated in FIG. 3;

FIG. 8 illustrates an exemplary magnetic sensing mechanism that can beused in the MEMS gyroscope illustrated in FIG. 3;

FIG. 9 shows an exemplary thin-film stack that can be configured into aCIP or CPP structure for use in the magnetic sensing mechanismillustrated in FIG. 8;

FIG. 10 illustrates an exemplary MEMS gyroscope that comprises multiplemagnetic sensing structures;

FIG. 11 illustrates an exemplary operation for detecting and measuringan angular velocity using a MEMS gyroscope illustrated in FIG. 3;

FIG. 12 illustrates temperature dependence of the coercivity of aferromagnetic thin film, wherein the ferromagnetic thin film can be usedin the signal sensor illustrated in FIG. 3;

FIG. 13 illustrates temperature dependence of the coercivity of aferromagnetic thin film, wherein the ferromagnetic thin film can be usedin the signal sensor illustrated in FIG. 3; and

FIG. 14 illustrates the temperature dependence of the magnetic exchangefield between a pining layer and a free layer, wherein the pinning layerand the free layer can be used in the signal sensor illustrated in FIG.3.

DETAILED DESCRIPTION OF SELECTED EXAMPLES

Disclosed herein is a MEMS gyroscope and method of using the same forsensing an angular velocity, wherein the MEMS gyroscope utilizes amagnetic sensing mechanism. It will be appreciated by those skilled inthe art that the following discussion is for demonstration purposes, andshould not be interpreted as a limitation. Many other variations withinthe scope of the following disclosure are also applicable. For example,the MEMS gyroscope and the method disclosed in the following areapplicable for use in accelerometers.

Referring to FIG. 3, an exemplary MEMS gyroscope is illustrated herein.In this example, MEMS gyroscope 106 comprises magnetic sensing mechanism114 for sensing the target angular velocity through the measurement ofproof-mass 112. Specifically, MEMS gyroscope 106 comprisesmass-substrate 108 and sensor substrate 110. Mass-substrate 108comprises proof-mass 112 that is capable of responding to an angularvelocity. The two substrates (108 and 110) are spaced apart, forexample, by a pillar (not shown herein for simplicity) such that atleast the proof-mass (112) is movable in response to an angular velocityunder the Coriolis effect. The movement of the proof-mass (112) and thusthe target angular velocity can be measured by magnetic sensingmechanism 114.

The magnetic sensing mechanism (114) in this example comprises amagnetic source 116 and magnetic sensor 118. The magnetic source (116)generates a magnetic field, and the magnetic sensor (118) detects themagnetic field and/or the magnetic field variations that is generated bythe magnetic source (116). In the example illustrated herein in FIUG. 3,the magnetic source is placed on/in the proof-mass (112) and moves withthe proof-mass (112). The magnetic sensor (118) is placed on/in thesensor substrate (120) and non-movable relative to the moving proof-mass(112) and the magnetic source (116). With this configuration, themovement of the proof-mass (112) can be measured from the measurement ofthe magnetic field from the magnetic source (116).

Other than placing the magnetic source on/in the movable proof-mass(1112), the magnetic source (116) can be placed on/in the sensorsubstrate (120); and the magnetic sensor (118) can be placed on/in theproof-mass (112).

It is also noted that the MEMS gyroscope illustrated in FIG. 3 can alsobe used as an accelerometer.

The MEMS gyroscope as discussed above with reference to FIG. 3 can beimplemented in many ways, one of which is illustrated in FIG. 4.Referring to FIG. 4, the proof-mass (120) is driven by capacitive, suchas capacitive comb. The sensing mode, however, is performed using themagnetic sensing mechanism illustrated in FIG. 3. For this reason,capacitive combs can be absent from the proof-mass (120).

Alternatively, the proof-mass can be driven by magnetic force, anexample of which is illustrated in FIG. 5. Referring to FIG. 5, the masssubstrate (108) comprises a movable proof-mass (126) that is supportedby flexible structures such as flexures 128, 129, and 130. The layout ofthe flexures enables the proof-mass to move in a plane substantiallyparallel to the major planes of mass substrate 108. In particular, theflexures enables the proof-mass to move along the length and the widthdirections, wherein the length direction can be the driving modedirection and the width direction can be the sensing mode direction ofthe MEMS gyro device. The proof-mass (126) is connected to frame 132through flexures (128, 129, and 130). The frame (132) is anchored bynon-movable structures such as pillar 134. The mass-substrate (108) andsensing substrate 110 are spaced apart by the pillar (134). Theproof-mass (112) in this example is driving by a magnetic drivingmechanism (136). Specifically, the proof-mass (126) can move (e.g.vibrate) in the driving mode under magnetic force applied by magneticdriving mechanism 136, which is better illustrated in FIG. 6.

Referring to FIG. 6, the magnetic driving mechanism 136 comprise amagnet core 138 surrounded by coil 140. By applying an alternatingcurrent through coil 140, an alternating magnetic field can be generatedfrom the coil 140. The alternating magnetic field applies magnetic forceto the magnet core 140 so as to move the magnet core. The magnet corethus moves the proof-mass.

The magnetic source (114) of the MEMS gyroscope (106) illustrated inFIG. 3 can be implemented in many ways, one of which is illustrated inFIG. 7. Referring to FIG. 7, conductive wire 142 is displaced on/inproof-mass 112. In one example, conductive wire 142 can be placed on thelower surface of the proof-mass (112), wherein the lower surface isfacing the magnetic sensors (118 in FIG. 3) on the sensor substrate(110, in FIG. 3). Alternatively, the conductive wire (142) can be placedon the top surface of the proof-mass (112), i.e. on the opposite side ofthe proof-mass (112) in view of the magnetic sensor (118). In anotherexample, the conductive wire (142) can be placed inside the proof-mass,e.g. laminated or embedded inside the proof-mass (112), which will notbe detailed herein as those examples are obvious to those skilled in theart of the related technical field.

The conductive wire (142) can be implemented in many suitable ways, oneof which is illustrated in FIG. 7. In this example, the conductive wire(142) comprises a center conductive segment 146 and tapered contacts 144and 148 that extend the central conductive segment to terminals, throughthe terminals of which current can be driven through the centralsegment. The conductive wire (142) may have other configurations. Forexample, the contact tapered contacts (144 and 148) and the centralsegment (146) maybe U-shaped such that the tapered contacts may besubstantially parallel but are substantially perpendicular to thecentral segment, which is not shown for its obviousness.

The magnetic sensor (118) illustrated in FIG. 3 can be implemented tocomprise a reference sensor (150) and a signal sensor (152) asillustrated in FIG. 8. Referring to FIG. 8, magnetic senor 118 on/insensor substrate 120 comprises reference sensor 150 and signal sensor152. The reference sensor (150) can be designated for dynamicallymeasuring the magnetic signal background in which the target magneticsignal (e.g. the magnetic field from the conductive wire 146 asillustrated in FIG. 7) co-exists. The signal sensor (152) can bedesignated for dynamically measuring the target magnetic signal (e.g.the magnetic field from the conductive wire 146 as illustrated in FIG.7). In other examples, the signal sensor (152) can be designated fordynamically measuring the magnetic signal background in which the targetmagnetic signal (e.g. the magnetic field from the conductive wire 146 asillustrated in FIG. 7) co-exists, while the signal sensor (150) can bedesignated for dynamically measuring the target magnetic signal (e.g.the magnetic field from the conductive wire 146 as illustrated in FIG.7).

The reference sensor (150) and the signal sensor (152) preferablycomprise magneto-resistors, such as AMRs, giant-magneto-resistors (suchas spin-valves, hereafter SV), or tunneling-magneto-resistors (TMR). Fordemonstration purpose, FIG. 9 illustrates a magneto-resistor structure,which can be configured into CIP (current-in-plane, such as aspin-valve) or a CPP (current-perpendicular-to-plane, such as TMRstructure). As illustrated in FIG. 9, the magneto-resistor stackcomprises top pin-layer 154, free-layer 156, spacer 158, reference layer160, bottom pin layer 162, and substrate 120. Top pin layer 154 isprovided for magnetically pinning free layer 156. The top pin layer canbe comprised of IrMn, PtMn or other suitable magnetic materials. Thefree layer (156) can be comprised of a ferromagnetic material, such asNiFe, CoFe, CoFeB, or other suitbale materials or the combinationsthereof. The spacer (158) can be comprised of a non-magnetic conductivematerial, such as Cu, or an oxide material, such as Al₂O₃ or MgO orother suitable materials. The reference layer (160) can be comprised ofa ferromagnetic magnetic material, such as NiFe, CoFe, CoFeB, or othermaterials or the combinations thereof. The bottom pin layer (162) isprovided for magnetic pinning the reference layer (160), which can becomprised of a IrMn, PtMn or other suitable materials or thecombinations thereof. The substrate (120) can be comprised of anysuitable materials, such as glass, silicon, or other materials or thecombinations thereof.

In examples wherein the spacer (158) is comprised of a non-magneticconductive layer, such as Cu, the magneto-resistor (118) stack can beconfigured into a CIP structure (i.e. spin-valve, SV), wherein thecurrent is driven in the plane of the stack layers. When the spacer(158) is comprised of an oxide such as Al₂O₃, MgO or the like, themagneto-resistor stack (118) can be configured into a CPP structure(i.e. TMR), wherein the current is driven perpendicularly to the stacklayers.

In the example as illustrated in FIG. 9, the free layer (156) ismagnetically pinned by the top pin layer (154), and the reference layer(160) is also magnetically pinned by bottom pin layer 162. The top pinlayer (154) and the bottom pin layer (162) preferably having differentblocking temperatures. In this specification, a blocking temperature isreferred to as the temperature, above which the magnetic pin layer ismagnetically decoupled with the associated pinned magnetic layer. Forexample, the top pin layer (154) is magnetically decoupled with the freelayer (156) above the blocking temperature T_(B) of the top pin layer(154) such that the free layer (156) is “freed” from the magneticpinning of top pin layer (154). Equal to or below the blockingtemperature T_(B) of the top pin layer (154), the free layer (156) ismagnetically pinned by the top pin layer (154) such that the magneticorientation of the free layer (156) is substantially not affected by theexternal magnetic field. Similarly, the bottom pin layer (162) ismagnetically decoupled with the reference layer (160) above the blockingtemperature T_(B) of the bottom pin layer (162) such that the referencelayer (160) is “freed” from the magnetic pinning of bottom pin layer(162). Equal to or below the blocking temperature T_(B) of the bottompin layer (162), the reference layer (160) is magnetically pinned by thebottom pin layer (162) such that the magnetic orientation of thereference layer (162) is substantially not affected by the externalmagnetic field.

The top and bottom pin layers (154 and 162, respectively) preferablyhave different blocking temperatures. When the free layer (156) is“freed” from being pinned by the top pin layer (154), the referencelayer (160) preferably remains being pinned by the bottom pin layer(162). Alternatively, when the free layer (156) is still pinned by thetop pin layer (154), the reference layer (160) can be “freed” from beingpinned by the bottom pin layer (162). In the later example, thereference layer (160) can be used as a “sensing layer” for responding tothe external magnetic field such as the target magnetic field, while thefree layer (156) is used as a reference layer to provide a referencemagnetic orientation.

The different blocking temperatures can be accomplished by usingdifferent magnetic materials for the top pin layer (154) and bottom pinlayer (162). In one example, the top pin layer (154) can be comprised ofIrMn, while the bottom pin layer (162) can be comprised of PtMn, viceversa. In another example, both of the top and bottom pin layers (154and 162) may be comprised of the same material, such as IrMn or PtMn,but with different thicknesses such that they have different blockingtemperatures.

It is noted by those skilled in the art that the magneto-resistor stack(118) is configured into sensors for sensing magnetic signals. As such,the magnetic orientations of the free layer (156) and the referencelayer (160) are substantially perpendicular at the initial state. Otherlayers, such as protective layer Ta, seed layers for growing the stacklayers on substrate 120 can be provided. It is further noted that themagnetic stack layers (118) illustrated in FIG. 9 are what is oftenreferred to as “bottom pin” configuration in the field of art. In otherexamples, the stack can be configured into what is often referred as“top pinned” configuration in the field of art, which will not bedetailed herein.

In some applications, multiple magnetic sensing mechanisms can beprovided, an example of which is illustrated in FIG. 10. Referring toFIG. 10, magnetic sensing mechanisms 116 and 164 are provided fordetecting the movements of proof-mass 112. The multiple magnetic sensingmechanisms can be used for detecting the movements of proof-mass 112 indriving mode and sensing mode respectively. Alternatively, the multiplemagnetic sensing mechanisms 116 and 164 can be provided for detectingthe same modes (e.g. the driving mode and/or the sensing mode).

By using the different blocking temperatures of the sensors as discussedabove with reference to FIG. 9, the reference sensor (150) and signalsensor (152) can be dynamically activated or deactivated for sensing thetarget magnetic field. For demonstration purpose, FIG. 11 shows anexemplary operation method of measuring a target magnetic field (e.g.from the wire 146 as illustrated in FIG. 7) by using the magnetic sensor(118 as illustrated in FIG. 8).

With reference to FIG. 7, FIG. 8, and FIG. 11, the wire can be set tothe OFF state by not driving a current through the wire at the initialtime T₀. The reference sensor can be set to the ON state. The ON stateof the reference sensor can be achieved through “freeing” the free layerof the reference sensor by raising the temperature of the pin layer thatpins the free layer of the reference sensor above its blockingtemperature (e.g. by applying a series of heating pulses or currentpulses) and driving a current through the reference sensor so as tomeasure its magneto-resistance. The signal sensor at this time can beset to the ON or any other suitable state, even though it is preferredthat the signal sensor can be set to the OFF state to avoid the magneticfield generated by the current driven through the signal sensor forsetting the signal sensor to the ON state. Because the wire is set tothe OFF state and no current is driven through the wire, the referencesensor measures the instant magnetic signal background at time T₀. Afterthe reference sensor finished the measurement, it locks its instantstate at time T₁ by for example, lowering the temperature of its top pinlayer (used for pinning the free layer) below its blocking temperaturesuch that the free layer is magnetically coupled to (thus pinned by) thetop pin layer. The reference sensor at this state is referred to as the“Lock” state.

At time T₁, the wire remains OFF and the signal sensor can be at anystate. When the reference sensor is stabled at the “Lock” state (e.g.finishes “locking” the state of its free layer), the wire is set to theON state at time T₂, by driving current with pre-defined amplitudethrough the wire so as to generate magnetic field. The current can be DCor AC. After the magnetic field generated by the wire is stabilized, thesignal sensor can be set to the ON state. Setting the signal sensor tothe ON state can be accomplished by raising the temperature of the pinlayer used for pinning the free layer of the signal sensor above itsblocking temperature so as to free the free layer. A current is driventhrough the signal sensor so as to measure its magneto-resistance.

After the signal sensor finished the measurement, it locks its instantstate at time T₃ by for example, lowering the temperature of its top pinlayer (used for pinning the free layer) below its blocking temperaturesuch that the free layer is magnetically coupled to (thus pinned by) thetop pin layer. The signal sensor at this state is referred to as the“Lock” state.

When the signal sensor finishes its locking at time T₃, the referencesensor and the signal sensor can output their measurements to so toobtain the magnetic field from the magnetic source attached to theproof-mass, thus extract the information of the movement of theproof-mass.

The reference sensor and the signal sensor can be connected by aWheatstone bridge, or can be connected directly to an amplifier or otherelectrical circuits to obtain the target magnetic field, which not bedetailed herein.

In the example discussed above, the reference sensor and/or the signalsensor can be configured to “lock” the status (e.g. the detectedmagnetic signal). This locking capability can be accomplished in manyways. In the following, such locking capability will be discussed withreference to signal sensor, and the reference sensor can be implementedin substantially the same ways.

In one example, the signal sensor can be configured to be comprised of astorage layer that comprises a ferromagnetic layer. The storage layer isconnected to electrical leads such that electrical current can beapplied through the storage layer. When current is applied, the storagelayer is heated, and its temperature can be elevated.

The material, as well as the geometry (e.g. the thickness) of thestorage layer can be configured such that at the elevated temperatureabove a threshold temperature, such as the Currie temperature, thestorage layer is capable of being magnetized by the target magneticsignal so as to accomplish the detection of the target magnetic signal.When the temperature of the storage layer is dropped to a temperaturebelow the threshold temperature, the storage layer “freezes” itsmagnetization states so as to accomplish its “locking” operation. FIG.12 illustrates such operation.

Referring to FIG. 12, the vertical axis plots the coercivity of thestorage layer (e.g. the ferromagnetic layer of the storage layer); andthe horizontal axis plots the temperature. The coercivity of the storagelayer (ferromagnetic layer) decreases with increased temperature. Atroom temperature RT, the storage layer has a coercivity that is higherthan the target magnetic signal H_(target), therefore, the storage layeris unable to detect the target magnetic signal. As the temperature ofthe storage layer increases, the coercivity of the storage layerdecreases. At the storing temperature (or the blocking temperaturewherein the signal storage layer transits from ferromagnetic toparamagnetic or super-paramagnetic), the coercivity of the storage layeris equal to or less than the target magnetic signal H_(signal) such thatthe storage layer is capable of being magnetized and thus detecting thetarget magnetic signal. After the detection, the temperature of thestorage layer can be decreased, by for example, removing the currentapplied through the storage layer (e.g. the ferromagnetic layer of thestorage layer). When the temperature of the storage layer is decreasedto a temperature below the storing temperature, the magnetization stateof the ferromagnetic layer (storage layer) is “locked” because thecoercivity of the storage layer is higher than the target magneticfield. With this mechanism, the storage layer accomplishes the “lockingprocess.”

The coercivity of a magnetic thin-film (layer) also varies with itsthickness, as diagrammatically illustrated in FIG. 13. The storage layercan have a thickness such that the coercivity of the signal-storagelayer is in the vicinity of the target magnetic field H_(target), suchas within a range of ±0.5%, ±1%, ±1.5%, ±2%, ±2.5%, ±3%, ±4%, ±5%, ±8%,±10% of H_(target). Especially when the storage layer has a thicknesssuch that its coercivity is higher than H_(target), a thermal layer canbe provided to adjust the coercivity of the storage layer.

In addition to utilizing the temperature dependence of coercivity of aferromagnetic layer, a magnetic coupling structure can be utilized toaccomplish the “locking” process, as illustrated in FIG. 14. Referringto FIG. 4, the signal sensor comprises free layer 172 and pinning layer170. The free layer (172) is a ferromagnetic layer, and is provide forresponding to the target magnetic field to be detected. The pinninglayer (170) is an antiferromagnetic layer, and is provided formagnetically pining the free layer (172) through the exchange magneticfield H_(exch). It is known in the art that the magnetic exchange fieldH_(exch) changes with temperature. When the temperature is higher thanthe blocking temperature T_(B) that characterizes the magnetic exchangefield between the free layer (172) and pining layer (170), the magneticexchange field between the free layer (172) and pining layer (172) isbroken, e.g. reduced to a level such that the free layer (172) andpinning layer (172) are magnetically decoupled. The free layer (172) isnot pinned by the pinning layer (172) at this temperature. By utilizingsuch magnetic-couple (pinning) and magnetic-decouple (unpinning), thesignal sensor comprising the free layer (172) and pining layer (170) canaccomplish the state “locking” process.

For example, when it is desired to detect the target magnetic fieldsignal, the signal sensor may elevate its temperature above the blockingtemperature T_(B) by for example, applying current through the freelayer (172) and/or the pining layer (170). The free layer (172) is thus“freed” and can be used for picking up the target magnetic field signal.When it is desired for the signal sensor to lock its detection, forexample, after the detecting the target magnetic signal, the signalsensor can decrease its temperature below the blocking temperatureT_(B). At a temperature below T_(B), the free layer (172) ismagnetically pinned by the pinning layer (172). The magnetic states ofthe free layer (172), which corresponds to the target magnetic fieldsignal, is thus “frozen” in the free layer (172).

It will be appreciated by those of skilled in the art that a new anduseful MEMS gyroscope has been described herein. In view of the manypossible embodiments, however, it should be recognized that theembodiments described herein with respect to the drawing figures aremeant to be illustrative only and should not be taken as limiting thescope of what is claimed. Those of skill in the art will recognize thatthe illustrated embodiments can be modified in arrangement and detail.Therefore, the devices and methods as described herein contemplate allsuch embodiments as may come within the scope of the following claimsand equivalents thereof. In the claims, only elements denoted by thewords “means for” are intended to be interpreted as means plus functionclaims under 35 U.S.C. §112, the sixth paragraph.

We claim:
 1. A method of measuring an angular velocity by using a MEMSgyroscope, the method comprising: generating a magnetic field that isassociated with a movement of a proof-mass of the MEMS gyroscope,wherein the magnetic field varies at a location of a magnetic sensor;detecting the variation of the magnetic field by the magnetic sensor;and extracting the angular velocity from the variation of the magneticfield.
 2. The method of claim 1, wherein the step of generating themagnetic field comprises: driving a current through a conductive wire soas to generating the magnetic field.
 3. The method of claim 2, whereinthe magnetic sensor comprises a spin-valve structure.
 4. The method ofclaim 2, wherein the magnetic sensor comprises atunneling-magneto-resistor structure.
 5. The method of claim 2, furthercomprising: before generating the magnetic field, detecting a magneticsignal from a background in which the magnetic field to be generatedco-exists; and wherein the step of extracting the angular velocityfurther comprises: extracting the angular velocity from said detectedmagnetic signal from the background and the variation of the magneticfield.
 6. A method of detecting a target angular velocity, comprising:providing a MEMS gyroscope that comprises a movable proof-mass, amagnetic source, and a magnetic sensor, wherein the proof-mass iscapable of moving in response to the target angular velocity under theCoriolis effect, and wherein the magnetic source is capable ofgenerating a magnetic field that varies with the movement of theproof-mass, and wherein the magnetic sensor is capable of detecting themagnetic field from the magnetic source; generating the magnetic fieldby the magnetic source; detecting a variation of the magnetic field bythe magnetic sensor; and extracting the angular velocity from thevariation of the magnetic field.
 7. The method of claim 6, wherein thestep of generating the magnetic field comprises: driving a currentthrough a conductive wire so as to generating the magnetic field.
 8. Themethod of claim 7, wherein the magnetic sensor comprises a spin-valvestructure.
 9. The method of claim 7, wherein the magnetic sensorcomprises a tunneling-magneto-resistor structure.
 10. The method ofclaim 7, further comprising: before generating the magnetic field,detecting a magnetic signal from a background in which the magneticfield to be generated co-exists; and wherein the step of extracting theangular velocity further comprises: extracting the angular velocity fromsaid detected magnetic signal from the background and the variation ofthe magnetic field.
 11. A method of measuring an angular velocity byusing a MEMS gyroscope, the method comprising: detecting a backgroundmagnetic signal by a reference magnetic sensor of the MEMS gyroscope;locking the state of the reference sensor after the detection of thebackground magnetic signal; generating a magnetic field by a magneticsource whose movement is associated with a proof-mass of the MEMSgyroscope; detecting the magnetic field from the magnetic source by asignal sensor; and calculating the angular velocity from the detectedmagnetic field.
 12. The method of claim 11, further comprising: lockingthe signal sensor after the step of detecting the magnetic field by thesignal sensor and before the step of calculating the angular velocity.13. The method of claim 12, wherein the step of generating the magneticfield comprises: driving a current through a conductive wire so as togenerating the magnetic field.
 14. The method of claim 13, wherein themagnetic sensor comprises a spin-valve structure.
 15. The method ofclaim 13, wherein the magnetic sensor comprises atunneling-magneto-resistor structure.